1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system, such as a printer, a copying machine or the like, where an image is recorded on a photosensitive body by scanning a surface of the photosensitive body with scanning lines comprising light beams which have been modulated with image data by a light-scanning unit, and particularly to a mounting structure of a light-scanning unit.
2. Description of the Related Art
As image forming apparatuses employing laser-beam scanning, a laser printer, a copying machine and a digital copying machine have been commercialized. In a field of such digital image recording apparatuses, high accuracy of assembling is required and respective elements or parts are unitized to improve productivity so that reduction in manufacturing cost and commercial price can be facilitated. Recently, in order to suppress a space for installing an apparatus to the minimum one, compact sizing or downsizing of the apparatus itself has been advanced.
In a currently popular color digital copying machine, after predetermined processing is performed on respective color separation images of an original input from a scanner, images are formed for respective colors by a plurality of image forming sections provided to corresponding to the respective colors and these images are sequentially transferred on a recording sheet of paper conveyed by a transferring/conveying belt in a superimposing manner to obtain one color image. In such a color copying machine, respective color images are reproduced faithfully and the respective color images are superimposed on the recording sheet with a high accuracy so that image representation included in the original image having no color shear can be reproduced faithfully without being injured.
However, when compact-sizing is facilitated, there occurs a problem that components or parts inside the apparatus are made close to one another to interfere with one another. Particularly, when the interior of the apparatus is elevated up to a high temperature due to heat generated from such a heat source as a fusing or fixing device, distortion, flexure, torsion or the like occurs in an optical device section due to thermal expansions of respective components, which results in adverse influence on an image recorded in an image writing step.
In order to overcome such an influence due to such heat, conventionally, when ozone, scattering materials and the like generated inside the apparatus are exhausted outside the apparatus through a filter, inside air including ozone, scattering materials and the like is exhausted to pass near to such a heat source as heat fixing means so that heated atmosphere is also exhausted together with ozone, scattering materials and the like. Such a countermeasure is not concluded to be sufficient as a countermeasure for reducing influences such as distortion, flexure, torsion and the like of devices inside the apparatus which are generated due to heat. Also, in a color copying machine having a plurality of light-scanning units corresponding to respective colors, since the respective light-scanning units are individually influenced adversely due to thermal expansion, when respective images are superimposed, color shear appears, thus, an adverse influence being remarkable.
Also, in an image forming apparatus wherein light-scanning recording is performed in a conventional electrophotographic system, for example, a digital copying machine or a printer, a drum-shaped photosensitive body or member which is an image carrier and a light-scanning unit are positioned with a predetermined positional relationship therebetween, and an image is recorded and reproduced by exposure-scanning a surface of the image carrier with light-beams which have been modulated on the basis of image data. In this light-scanning recording apparatus, the positional relationship between the photosensitive body and the light-scanning unit influences the quality of image recorded and reproduced, and further distortion or the like generated in optical elements or parts of a scanning optical system which are optical components also influences the image quality of an image recorded and reproduced.
In Japanese Examined Utility Model Publication JP-Y2 3-10486 (1991) which is one conventional art, for example, a semiconductor laser device, a multi-facet mirror which is a polygon mirror, lenses, mirrors and the like, these members constituting a scanning optical system of a laser scanning device, are respectively fixed at predetermined positions on a supporting body or member which is a base body. For strengthening the base body itself and suppressing the coefficient of thermal expansion thereof, the base body is a molted product made of synthetic resin mixed with glass fibers, for example, synthetic resin of polycarbonate, polyphenylene sulfite, epoxy, or polyester systems and is formed in a box-shape. This supporting body is mounted on a metal base such as an aluminum plate, a steel plate or the like, and a surface of the photosensitive body is light-scanned to record an image. The supporting body for supporting the semiconductor laser device, the multi-facet mirror, the lenses, the mirrors and the like which constituting the this scanning optical system must be mounted on the metal base in a stable state. Otherwise, distortion occurs in the supporting body and it becomes impossible to record and reproduce an image faithfully.
In general, in view of workability for assembling an apparatus, a manufacturing technique is conventionally performed that elements to be unitized are preliminarily prepared as respective units and, after assembling is performed for each unit, these units are assembled in the apparatus. Here, it is best preferable that respective optical parts for the light-scanning unit which may influence the image quality of an image recorded and reproduced are firmly assembled at predetermined positions on the supporting body to complete these parts as one unit. Respective completed units are not used immediately for assembling an apparatus as they are, but these units are assembled as an apparatus via transportation, parts management and the like. Therefore, each unit must have a structure which can bear any environment such as strength, thermal expansion or the like of the supporting body. However, there is a limitation in improvement in the strength of the supporting body in view of cost, size, weight and the like.
In Japanese Unexamined Patent Publication JP-A 7-325238 (1995) which is another conventional art, there has been disclosed a configuration for positioning and adjusting a light-scanning unit to an apparatus body. The configuration will be briefly explained. A photosensitive body is rotatably supported between a front side plate and a rear side plate, and further a top plate, a pair of shafts made of a stainless steel and a light-scanning unit held on the shafts are supported above the photosensitive body. In the light-scanning unit, a holding member provided at a lower face thereof is engaged with the pair of shafts, and the holding member is engaged with the shafts made of a stainless steel and moreover, a boss fixed to the light-scanning unit in a downwardly projecting manner is engaged with an elongated hole formed in the top plate of the apparatus body. Adjustment of a scanning line position of a laser light is performed by moving ends of the shafts positioned at a side of the rear side plate. The light-scanning unit is mounted and supported on the shafts provided at the apparatus body and the shafts are shifted or displaced so that adjustment of an image writing position of the light-scanning unit to a drum surface of the photosensitive body is performed.
In this conventional art, since the light-scanning unit is mounted and supported on the pair of shafts, the degree of freedom is low due to a positional relationship between the light-scanning unit and the supporting points of the light-scanning unit on the shafts. Also, when distortion due to heat occurs in the light-scanning unit, influence of such a distortion on the image quality of an image becomes large. Therefore, it is necessary to increase the strength of a frame of the light-scanning unit in order to suppress distortion occurring in the light-scanning unit as much as possible, but such an increase in strength results in increase in manufacturing cost and weight of the light-scanning unit.